--- In harryproa@yahoogroups.com.au, "bjarthur123" <bjarthur123@...> wrote:
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> i understand that it's about L/D upwind, and L downwind.
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> i've been told, here and elsewhere, that L/D is better for a unarig than main+jib. can't say i have a firm intuitive grasp as to why though.
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> given these bits as true, why did both BMW-Oracle and Alinghi have bermudan rigs in the last america's cup? the only rule was 90 ft long to my knowledge. would they have been faster with unarigs?
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> my guess is that the mast is heavier aloft on a stayed unarig b/c it's longer for the same righting moment. better it seems to put sails on both the aft and fore sides of the mast to maximize the sail area to mast weight ratio.
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> no intuition here either though, just data from what designers have done in races without rules. participants in "the race" all had bermudans as well, except for team phillips. cat boats are extraordinarily rare. why?
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> not trying to be sarcastic or difficult here. really am just curious to understand how it all works.
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> on an unstayed rig it might all be different. the added compression from the forestay calls for a heavier mast. fore boom adds weight. is it enough to offset the benefits? i dunno.
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> sure would be nice to have a two boat campaign to figure it all out.
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> ben
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It is about L/D upwind, but first you need enough area, and you must not capsize too easily. Hence the popularity of genoas which are lots of area down low. If you have a very high righting moment, a very light boat and can get sail area up high in the light and remove it in the heavy, then a una rig is more efficient. At least I hope it is as this is the thinking behind the telescoping una rig.
Unstayed gets comparatively lighter than stayed as the length increases, as the top part sees reducing loads. However, to keep the top stiff enough is a problem, especially if you are thickness limited which is more of a problem with a round tube than with a wing section. Until you get beyond Airbus 380 wing lengths, it seems to be surmountable.
rob
rob